The isomerization of paraffinic hydrocarbons having from 4 to 6 carbon atoms is practiced to create more highly branched isomers from the normal or straight-chain hydrocarbons. One purpose of an isomerization process is to provide high octane blend components for refined gasoline. The isomers can also be used in the production of other products or used as solvents. For the isomerization of normal butane, it is generally desired to produce an isobutane product that can be used as a feedstock in the manufacture of methyl tertiary butyl ether (MTBE) or in the manufacture of an alkylate product produced by the alkylation of isobutane with olefin hydrocarbon compounds. The isomerization of pentane, hexane, and refinery light naphtha mixtures is also practiced in order to produce high octane gasoline blending components. The isomerication of normal or straight-chain hydrocarbons can significantly improve the overall octane number of the gasoline pool produced from petroleum refining operations.
In the isomerization of paraffin hydrocarbons, it is desirable to isomerize a hydrocarbon-containing feedstream that contains a minimum amount of sulfur compounds and water because of their contaminating effects upon the isomerization catalyst. Generally, water serves as a poison to isomerization catalysts, and sulfur compounds act as temporary poisons that inhibit catalyst isomerization activity. The larger the concentration of water and sulfur compounds within a hydrocarbon-containing stream, the lower the attainable conversion per pass of the normal hydrocarbons to their corresponding isomer compounds, consequently resulting in lower product yields.
Recent developments in the process for producing linear low density polyethylene have resulted in the requirement that an extremely high purity, essentially contaminant-free isopentane product be utilized as a solvent. Generally, this high purity isopentane requires that its sulfur content be less than 1 part per million by weight (ppmw) and that the water content be less than 10 ppmw. Because of the more rigorous product specification for isopentane that is to be used in the production of linear low density polyethylene, it becomes more difficult and more costly to produce. While there are a number of commercial processes which can be used for the production of this high purity product, many of them are costly to operate and involve high initial capital costs.
There are many methods known in the art for removing water and sulfur compounds from hydrocarbon-containing feedstreams. One such process involves the caustic treating of a hydrocarbon stream to remove sulfur compounds as is described in U.S. Pat. No. 4,562,300 and the references cited therein. One difficulty with the caustic treating method referred to above is that disulfide compounds are produced which have a slight solubility in the hydrocarbon-containing stream being treated and, because of this slight solubility, small quantities of disulfide compounds remain in the treated hydrocarbon-containing stream. Due to the slight concentration of disulfide compounds in the treated hydrocarbon-containing stream when it is charged to an isomerization reaction zone, the isomerization catalyst activity is inhibited resulting in a lower conversion and lower product yield. Therefore, the removal of the small quantities of contaminating sulfur compounds from the hydrocarbon-containing feedstream prior to charging the feedstream to an isomerization reaction zone can substantially improve catalyst activity and conversion. Additionally, the removal of the disulfide compounds from the treated hydrocarbon-containing stream prior to charging the stream to an isomerization reactor can result in providing a high purity isomerate that contains minimal quantities of sulfur compounds.
It is, therefore, an object of this invention to provide an improved method and apparatus for removing contaminating quantities of water and sulfur compounds from hydrocarbon-containing streams.
A further object of this invention is to provide method and apparatus for enhancing the activity of isomerization catalyst in order to improve the conversion of straight-chain hydrocarbons to their corresponding branched chain isomers and to increase product yield.
A still further object of this invention is to provide method and apparatus for producing at low cost a treated hydrocarbon-containing stream suitable for isomerization.
Another object of this invention is to provide a method for producing an essentially contaminant-free, high purity isomerate product suitable for use in the production of linear low density polyethylene.